This invention relates to thermal protective compositions which form chars when exposed to fire or other thermal extremes. The invention is particularly well suited to use in solvent-based, thin-film intumescent coatings for substrates, but its usefulness is not limited thereto.
Various compositions are known which provide protection against fire and other thermal extremes, such as temperatures above about 300.degree. C. Some of the compositions are foamed inorganic passive insulative compositions which protect merely by their low thermal conductivity and their thickness as applied. These include, for example, foamed cement or intumesced silicates. The present invention is not concerned with such systems, but with systems which include a polymeric binder and which form a char when exposed to fire or hyperthermal conditions. The char-forming compositions may operate by various modalities. The compositions may be used in various forms, including thick film (mastic) coatings, thin film coatings, castings, extrusions, and others. The compositions may include organic or inorganic binders and various additives. Upon exposure to heat the compositions slowly lose weight as portions of the composition are volatilized, and a char is formed which provides a measure of protection against the transfer of heat energy. Eventually, the char is consumed by physical erosion and by chemical processes, primarily oxidation by oxygen in the air and by free radicals produced by the coating or otherwise in a fire environment, and protection is lost. The length of time required for a given temperature rise across a predetermined thickness of the composition, under specified heat flux, environmental, and temperature conditions, is a measure of the effectiveness of the composition in providing thermal protection.
When subjected to fire or other hyperthermal conditions, different coatings behave differently.
Ablative coatings swell to less than twice their original thickness. They provide limited passive thermal protection, but they tend to produce dense chars having good physical and chemical resistance.
Intumescent coatings swell to produce a char more than five times the original thickness of the coating. This char provides an insulative blanket which provides superior thermal efficiency, but at the cost of some of the physical and chemical properties of the ablative coatings. The char of the intumescent materials tends to form coarse and irregular cell structures, cracks, and fissures as it expands, and the char may not expand uniformly at corners, leaving areas where the char provides far less protection than the average thermal protection of the underlying structure. Examples of the intumescent systems include silicate solutions or ammonium phosphate paints or mastic compositions such as those disclosed in Nielsen et al., U.S. Pat. Nos. 2,680,077, Kaplan, 3,284,216, or Ward et al., 4,529,467.
A third type of char-forming coating is disclosed in Feldman, U.S. Pat. No. 3,849,178. When subjected to thermal extremes, these compositions both undergo an endothermic phase change and expand two to five times their original thickness to form a continuous porosity matrix. These coatings tend to be tougher than intumescent coatings. They provide far longer thermal protection than ablative coatings, frequently longer than intumescent coatings, in part because the gasses formed by the endothermic phase change provide active cooling as they work their way through the open-cell matrix. These coatings may also have a tendency to crack and form voids and fissures.
The present invention relates primarily to intumescent systems, particularly thin-film intumescent coating systems, i.e., those having a thickness as applied of less than five millimeters. Several aspects of the invention, however, are also applicable to thick film intumescent compositions and to the Feldman type compositions which undergo an endothermic phase change and swell two to five times their original thickness. Some aspects of the invention are also applicable to ablative char-forming coatings.